The present invention relates to a system which may be used with conventional disk storage equipment used for data handling in computer systems to allow operation in a continuous data acquisition or transmission mode. More specifically, this application relates to an invention whereby concentric servo tracks used in conventional data processing disk drive equipment may be used to operate the data heads in a spiral fashion with respect to the disk medium. Thus, the entire magnetic surface of the disk medium may be covered with a continuous playback or recording of data in a spiral tracking mode analogous to the manner in which a phonograph needle operates on a phonograph record.
As is well known, conventional data processing disk storage equipment operates with concentric servo tracks that close upon themselves arranged on the surface of a disk recording medium. These concentric servo tracks may occupy one or more disk surfaces in a disk pack while the other surfaces are for the purpose of receiving and replaying recorded data. Servo heads are used in such conventional devices to follow the position of a prerecorded servo track and thus operate a movable head positioning device so that the data heads are always located over a particular data track, corresponding to the servo track, in exactly the same position. This allows the device to efficiently retrieve and access data which has previously been stored. However, as is also well known, once data has been recorded or received from a particular data track, there is a certain interval of time during which the data head must be positioned to the next data track during which no information may be either received and stored or read and delivered. This interval of time, although short by human standards, is comparatively long with respect to the rate at which data may be entered into or retrieved from the system. It has been thought that if the technology and the high data rates available in modern computing equipment were available to read a single stream of data in or out on a continuous basis over an entire disk that greater usefulness can be obtained from such equipment. However, because of the fact that such equipments are designed for use with concentric servo tracks and appropriate control systems are already built into such devices based on concentric servo track design, extensive redesign would be required to make such units operate on spiral data tracks. Thus, it would be desirable to have a device which could allow the reading or writing of data in a continuous rotational spiral mode using as a position reference conventional concentric servo tracks already familiar to such equipment.
The prior art shows several systems using spiral tracks for video frequency systems such as that shown in U.S. Pat. No. 4,022,968. However, the system shown in that patent is controlled for timing purposes by a tachometer disk. Thus, the system utilized in that patent obtains certain information from a secondary disk, called a tachometer disk, but does not use or have such information based on any system of using concentric servo tracks.
One of many patents relating to the operation of the conventional concentric track servo and data disk storage systems is U.S. Pat. No. 3,838,457. This patent shows that the state of the art is well developed in computing systems for head positioning using concentric data tracks based on concentric servo tracks.
With respect to the present invention, it is not important which particular head positioning system based on concentric servo tracks is being used. It is necessary that a tracking system be used which provides a head error position signal which continuously gives a unique error position signal with respect to a particular servo track from the point where the head is positioned halfway between the given servo track and an outer servo track to the position where the servo head is positioned halfway between the given servo track and an inner servo track. That is, the servo tracking system employed in order to allow a unit to adapt to the present invention must be one such that when the servo head is passed continuously radially over a rotating servo disk having servo tracks thereon a continuous signal is generated indicating error position with respect to servo tracks. Generally, such an output will tend to consist of a triangular head position error signal which may be truncated at the peaks of the triangular wave forms but which is piecewise continuous.
Referring again to U.S. Pat. No. 3,838,457, other embodiments of the present invention may also be employed which work with a servo positioning signal or position error signal which varies with respect to centering over each individual servo track but which varies over a number of tracks as shown in the patent.
One further example of a suitable system for generating a head position error signal in a data processing system is that shown in U.S. Pat. No. 3,534,344.
U.S. Pat. No. 3,518,366 illustrates a slow motion picture video system based on magnetic disk storage elements. This patent shows a mechanical system for causing a transducer arm assembly to move a recording and playback device in a spiral fashion over a rotating disk. This type of machine can only operate in what will be called a "serial" fashion whereby a replay commences at the beginning of the recorded material, although the playback can be terminated at any desired place. This is distinct from a random access feature which allows the playback to commence at any selected portion of the recorded material. Furthermore, mechanical repositioning systems are subject to problems with respect to accurately and repeatedly returning to the same precise start position.
In the computer disk storage technology art, the improvement realized by the development of concentric data tracks and concentric servo tracks on the storage medium was principally that the data could be accessed in a random fashion by accessing or addressing any preselected data track with the selection of the appropriate servo track. This allowed significant improvement over other types of memory where all of the data had to be searched in order to read a particular desired fragment. Thus, the use of concentric data tracks combined with means for reading and writing on a continuous or serial spiral data track allows for both uninterrupted reading and writing of data as well as random access to a particular portion of the recorded data by reference to a particular servo track. In addition, a single disk can have a certain portion used in a spiral tracking mode and another portion used in a conventional mode.
U.S. Pat. No. 3,757,030 shows a video buffer memory with spiral tracking based on a scheme using a rotating disk and one or more transducer heads rotating on a parallel offset axis with respect to the rotational axis of the disk. This patent shows the complexity necessary to implement a mechanical system while the present invention is based on an electronic system design which may be used with otherwise conventional equipment.